1. Technical Field
This invention relates to a pressure operated fuel cutoff valve for a unit fuel injector for an internal combustion engine, of the type having an open nozzle and a cam driven reciprocating injector plunger.
2. Description of Related Art
Open nozzle unit fuel injectors are widely utilized because of their ability to achieve desired performance goals while being relatively less complicated and less expensive to manufacture than unit injectors of the closed nozzle type (i.e., unit injectors having pressure operated, normally closed tip valves). Fuel injectors of the open nozzle type often operate on the "pressure/time" principle developed by the assignee of this application, Cummins Engine Company, Inc. (see U.S. Pat. Nos. 3,351,288, 3,544,008, and 4,471,909). In a pressure/time fuel injector, fuel is metered into the injection chamber of each injector through a restricted metering orifice. The time during which each feed orifice is open and the pressure within the fuel supply line or common rail combine together to control the quantity of fuel metered for injection during each injection cycle. In systems of this type the pressure level of fuel supplied to each injector is caused to be a function of engine load. During low load or idling speed the pressure in the fuel supply line will be low, in contrast to a high load engine condition in which the fuel supply line pressure will be high.
Although this fuel supply system is widely used, problems caused by engine operation at low load or idling speed. In particular, white smoke, unburned hydrocarbons and injector carboning can occur under low load or idling speed of an engine equipped with conventional open nozzle injectors. White smoke is a condition that results on engine start-up or low-load motoring conditions due to improper combustion of fuel because of insufficient compression or temperature levels. In addition, certain precautions must also be undertaken in open nozzle type injectors to minimize the entry of combustion gases and other unwanted substances into the fuel supply system (i.e., a condition known as blowback).
Attempts have been made to provide satisfactory operation of fuel injectors at engine low load or idling speed by providing a pressure control valve in the fuel supply line leading to the injector, such as disclosed in U.S. Pat. No. 2,922,581 to Garday. In particular, the '581 patent discloses a valve with a pressure reduction capability located between the fuel supply pump and the fuel supply delivery line. The pressure reducing effect is attained by the use of variable restrictive passages to produce a uniform reduction in delivery line pressure throughout the full range of engine operation (i.e. from full load to idling speed). Although the purpose of a uniform reduction of delivery line pressure is to obtain more accurate timing and metering of fuel at idling speed, the disclosed arrangement is designed for use on a closed nozzle injector wherein the high pressure pump is separated from the injection nozzle by the delivery line, thereby requiring the delivery line to operate periodically at very high injection pressure. Accordingly, the pressure reducing feature is designed to deal with problems associated with high injection pressure transmitted over relatively long distances to improve the operation of a normally closed injector nozzle having a pressure operated tip valve. Such problems do not exist in open nozzle unit injectors. The '581 patent does not suggest how to minimize the problems associated with low load engine operation, such as white smoke, unburned hydrocarbons and injector carboning in cam operated, open nozzle unit injectors connected with low pressure fuel supply lines.
Many of the fuel injectors currently available attempt to prevent the entry of unwanted substances into the fuel supply. These attempts have generally been characterized by the use of a spring-biased check valve positioned in the fuel supply passage which is responsive to changes in the relative pressure upstream and downstream of the valves. U.S. Pat. Nos. 4,129,253, 2,285,730 and 3,355,108 each show such a check valve located upstream from the injection chamber in the fuel supply passage.
The upstream check valve placement is somewhat effective in reducing unburned hydrocarbons and blowback, however, these undesirable conditions may still exist and must be further minimized in order to achieve the efficient engine operation that is required to satisfy the increasingly higher performance goals of engine manufacturers. Further, use of check valves does not effectively prevent fuel from entering the injection chamber after the engine has been shut off, resulting in diesel engine motoring or run on, and fails to provide satisfactory fuel injector operation at engine low load or idling speed by reduction of white smoke.